Photolithography is widely used to form patterns on semiconductor wafers during fabrication of integrated circuits. A semiconductor wafer 110 (FIG. 1) including an optional topmost layer 115 is coated with a photoresist layer 120. Photoresist 120 is irradiated from a light source 130 through an annular aperture 160. A mask or reticle 140 is placed between source 130 and photoresist 120. Binary mask 140 carries a pattern consisting of opaque and clear features. This pattern defines which areas of photoresist 120 are exposed to the light from source 130. After the exposure, the photoresist 120 is developed so that some of the photoresist is removed to uncover the underlying surface of layer 115 on substrate 110. If the photoresist is “positive,” then the photoresist is removed where it was exposed to the light. If the photoresist is “negative,” the photoresist is removed where it was not exposed. In either case, the remaining photoresist and the exposed (uncovered) areas of substrate 110 reproduce the pattern on mask 140. The wafer is then processed as desired (e.g., the exposed areas of layer 115 and/or substrate 110 can be, for example etched, coated, plated, or implanted with a dopant, among other possibilities.)
There is a trend in the semiconductor industry to have ever smaller feature sizes, while at the same time there is a desire to use existing photolithography equipment for new generations of semiconductors. In general, the minimum feature width of the patterned area is limited by the wavelength of the particular light source used, due to diffraction effects. That is, light diffracts around the edges of an aperture, so that the rays spread out from the source before being incident on the substrate. Therefore, the feature dimensions become poorly defined, when the aperture is smaller than, or on the order of, the wavelength of the light used to illuminate the pattern.
A number of techniques exist for improving the shape of the edge features which determine the minimum feature dimension which a particular light source is capable of creating on the substrate. One technique involves using a half-tone mask, which employs areas which are partially transmissive, to create destructive interference at the boundaries of the illuminated feature. The interference causes a sharpening of the feature boundary, compared to what its shape would be using a binary mask, that is, one with areas that are either fully transmissive or fully opaque. Half-tone masks, however, are about twice as expensive as binary masks.
Accordingly, because of the need to create ever smaller integrated circuit (IC) devices, a method that can further reduce the feature size that can be made by a given light source on a substrate would be of great commercial benefit.